Generally, cemented tungsten carbide parts are made from powders of WC and Co mixed with an organic binder, such as wax, which are subsequently pressed and sintered. The binder is added to facilitate, for example, the flowability and cohesiveness of a part formed from the powders. To ensure a homogeneous mixture, the WC, Co and binder are typically mixed (e.g., ball or attritor milled) in a liquid. The liquid is generally a flammable solvent, such as heptane, to decrease the tendency for the WC to decarburize and for the WC and Co to pick up oxygen, for example, when mixed in water or air. The decarburization of the WC and introduction of excessive oxygen must be avoided because undesirable phases in the cemented carbide tend to occur, generally causing reduced strength.
Unfortunately, the use of a flammable solvent requires significant safety, environment and health precautions, resulting in a significant amount of cost to produce the pressable powder. To avoid some of these problems, WC particles greater than about 1 micrometer in diameter with cobalt and binders have been mixed or milled in water (U.S. Pat. Nos. 4,070,184; 4,397,889 4,478,888; 4,886,638; 4,902,471; 5,007,957 and 5,045,277). Almost all of these methods require the mixing of the WC powders with just the organic binder and, subsequently, heating the mixture until the binder melts and coats all of the WC particles before milling with Co in water.
Smaller WC particles (e.g., less than 0.5 micrometer in diameter) are now being used to increase the strength and hardness of cemented tungsten carbide parts. However, because of the increased specific surface area (m.sup.2 /g) of these WC powders, the avoidance of oxygen pickup has become more difficult. Consequently, the use of these smaller particles has tended to require the milling time to be longer to ensure a uniform mixture of WC with Co, exacerbating the problem of oxygen pick up. Because of these problems, these small powders, generally, are always processed in a solvent, such as heptane.
Thus, it would be desirable to provide a method to form a pressable powder that avoids one or more of the problems of the prior art, such as one or more of those described above.